The terms "stratigraphic accumulations" and "stratigraphic traps" are used herein in their broadest sense: traps that do not rely on structural or tectonic processes to bring about closure. The relevant process must therefore involve changes in the permeability and porosity of the host sediments, either by sedimentation or by metamorphic processes, say (1) silting of a sandstone body until the pore spaces are insufficiently large to allow the escape of the hydrocarbons; and (2) action of magnesium-bearing salts of, say, groundwater on limestone causing the transformation of the latter to dolomite which have a lower porosity than that of calcite again prevents the escape of the oil and gas.
Since such traps are not usually associated with large acoustic contrasts, mapping by conventional methods (such as by measuring changes in the arrival time of the associated reflection) cannot be done. Hence, indirect analytical methods are required. For example, time differences and changes in reflection wave shapes of traces associated with the interval of interest, are most often used, but with limited success due, inter alia, to the fact that resolution requires that the compared traces be essentially distortion-free, after processing has occurred.
The term "distortion-free, after processing" to describe the final records relates to the fact that there should be no undesired change in waveform either of the recorded field signals or of the final data after processing has been completed.
Prior to my invention, such distortion could be brought about (i) by the use of sign-bit recording techniques in the field as described hereinafter, (ii) by data processing procedures as where power spectrum of the collected data is provided using conventional processing methods, or (iii) accidentally, where components of the system do not linearly interface the input and output signals, as when the injected seismic signal exceeds the dynamic limits of the recording system so that signal clipping invariably results.
Doty et al, U.S. Pat. No. 2,688,124 issued Aug. 31, 1954 for "Method and Apparatus for Determining Travel Time of Signals" describes the well-known Vibroseis.RTM. system of Continental Oil Company. In such a system, seismic waves are generated by mechanical vibrators on the earth's surface. Each of the vibrators is firmly anchored to the earth, by the combined weight of the source. Peak forces in the neighborhood of 10 to 20 tons (and up to 36 tons) can be developed by the rapid non-explosive interaction of the base-plate and piston system of each vibrator. Consequently, the weight of each vibrator is proportionally large to maintain the desired continuous vibrator-earth contact during operations. The waves sent into the earth consist of long sinusoidal wave trains of predetermined frequency and time duration characteristics rather than the much sharper wave impulses sent into the earth by the explosive sources used prior to the Vibroseis.RTM. system, or by "weight drop" methods including those provided by various impulse coded systems, e.g., the so-called pulse-coded techniques.
There may be some confusion as to the differences of the signals produced by the Vibroseis.RTM. system and those produced by impulsive sources such as provided by exploding dynamite exploding mixtures of propane and air, or by "weight drop" methods including pulse-coded techniques.
It is well known that the capacity of any signal (including seismic signals) to carry information can be measured in a manner analogous to determining the volume of a container. Since volume is the product of height times width times length, similarly information capacity of a signal is related to a product of amplitude, frequency bandwidth and the length of the signal.
Dynamite as an seismic energy source produces an input signal having considerable amplitude (height), and bandwidth but has very short length. On the other hand, "non-impulsive" vibrations generated in the manner of a Vibroseis.RTM. system have limited amplitude, but such is compensated for by the long length of the input signal and a faithful continuous reproduction of the control signal over the frequency spectrum of interest. That is to say, in the Vibroseis.RTM. system the amplitude and phase spectra is carefully and continuously controlled so that the resulting energy spectra changes smoothly as a function of time. Thus, a smoothly varying output of desired frequency and duration characteristics is provided in contradistinction to the binary-coded (ON-OFF) squarewave output generated by pulse-coded methods in which the energy per blow is substantially constant and cannot be so controlled.
A further essential part of the Vibroseis.RTM. system lies in the processing of the received data to produce records that tend to show short pulses representing reflections from subsurface interfaces. Such responses are provided by cross-correlating the recorded representation of the vibratory waves sent into the ground with the recorded representation of the waves received subsequently.
The use of cross-correlation as taught by Doty et al and many others since, has now become so well known in vibratory seismology that it will be presumed to be well known in the following parts of the present specification; and the description will concern itself only with differences from the prior art.
Erich, U.S. Pat. No. 4,234,053 for "Seismic Exploration Method Using a Rotating Eccentric Weight Seismic Source", describes an exploration method in which a rotating eccentric weight source is used (as a power impactor) to transmit a coded, non-Gaussian impulse input signal into the earth on a substantially constant energy per blow basis. An improved representation of the pulsed input signal is correlated with the raw seismic data to provide the field record of interest. But since the impulsive source is also only discontinuously coupled to the earth (i) the interaction of the mass of the eccentric weight source with the spring constant of the earth produces an output dominated by low frequency components and (ii) the pulse shape of the output can vary nonlinearly with time. Hence such system is limited to those uses where a conventional Vibroseis.RTM. system cannot be employed.
Multi-array use of such sources is likewise limited.
Martin et al, U.S. Pat. No. 4,058,791 issued Nov. 15, 1977 "Method and Apparatus for Processing Seismic Signals from Low Energy Sources" is directed to an effort to solve the growing problem of handling all the information collected in a modern seismic survey. It is now desired to collect information from hundreds, and sometimes even thousands, of receivers, feeding into tens, and sometimes even hundreds, of recording channels. Martin et al recognize that some essence of the seismic information is preserved if only the algebraic signs of the incoming signals, and not the full waveforms are recorded. Using information channels that need to handle only sign-bits makes it possible to use several times as many channels for the same recording and processing capacity.
Also, Martin et al observed that in some of their vibratory seismic work, that when sign-bit representations of the source waves were cross-correlated with sign-bit representations of the received waves, the resulting cross-correlation functions appear to be similar to cross-correlation functions from full waveform inputs, provided that the resulting correlation functions are "common depth point stacked" to a high multiplicity ("the CDP fold is at least 40"). However, it is to be particularly noted that Martin et al use a conventional "chirp" source signal to generate vibrations. Furthermore, Martin et al indicate that where their stacked final records appeared similar to conventional stacked records using 16-bit recording, they were referring to work of their predecessors, such as Fort et al, U.S. Pat. No. 3,883,725, issued May 13, 1975, "Data Compositing and Array Control System", who added certain "shifting functions" to the received signals before the received signals were clipped. The requirement for high order stacking is objectionable because the large number of information channels required to produce a single stacked output trace tends to cancel out the very advantage for which sign-bit recording is used, its channel-capacity economy. A large number of low capacity channels can require just as much recording and processing capacity as a small number of high capacity channels. Addition of the "shifting functions" is objectionable. It does not improve records in the general case, even though it may have some value in certain limited circumstances (e.g., with low signal-to-noise-ratio signals). So the similarities noted by Martin et al between the cross-correlation function from their sign-bit recordings and cross-correlations from full waveform recordings depended on special circumstances not desirable to produce, or to encounter, in general seismic exploration work.
There is a further disadvantage of the Martin et al technique; they had no measure of the similarity they noticed. The results could not be stated in mathematical terms which would indicate how much information had been discarded in the clipping operation (the conversion to sign-bits) and whether or not the discarded information was essential.
Another relevant patent is that of Crook et al, U.S. Pat. No. 3,264,606, Aug. 2, 1966 "Method and Apparatus for Continuous Wave Seismic Prospecting" which teaches driving of vibratory sources (in conjunction with conventional full-wave recording equipment) with pseudo-random codes which, although differing in detail from the preferred codes prescribed here, does share the desirable generic property of "a code sequence which may be represented as a reference time series having a unique auto-correlation function comprising a single major lobe having no side lobes of greater amplitude than the side lobes of the auto-correlation function of statistically unrelated noise components of the composite signal detected at said detecting location" (Column 13, lines 32-44).
In my parent application for "Seismic Exploration Using Vibratory Sources, Sign-bit Recording, and Processing that Maximizes the Obtained Subsurface Information", filed Feb. 1, 1981, Ser. No. 177,689 assigned to the assignee of the present application, I describe a non-impulsive vibratory system that uses a class of vibrator signals best characterized as Gaussian, zero mean, and stationary, in conjunction with sign recording of both the injected and received vibrations at the sources and receivers. The stated advantages relate to the channel-capacity economy of sign-bit recording (at both the sources and receivers), and to the distortion-free quality of the final processed records.
I have now discovered that use of the above-class of vibrator signals not only does not sacrifice information in the final processed records nor reduces their distortion-free quality, (even though the data is collected by sign-bit recording methods) but also such type of vibrator signals also favorably impacts all classes and types of non-linear seismic recording and processing operations (typically classed as zero-memory, non-linear operations, or ZNL's).
Aside from the above, a paper of A. B. Cunningham, Geophysics, December 1979, Vol. 44, No. 12, pages 1901 et seq for "Some Alternate Vibrator Signals" works out in mathematical detail expected types of cross-correlation functions from various types of vibrator sweeps, including certain types of pseudo-random sweeps, but not in the context used herein.